Recently, to reduce costs for the development and the operation management of an information system, an operation management type called a cloud not for maintaining the information system by itself but for utilizing an information system provided by another organization attracts lots of attention. In the meantime, as an organization that manages an information system is different from an organization that utilizes an information system in a cloud, it is difficult to take measures to prevent an information leak and others, cause investigation after an accident occurs, measures to prevent reoccurrence and others by only the management organization of the information system. Therefore, for measures to prevent an unauthorized leak of data, is required to utilize cryptography and to secure confidentiality of data beforehand.
In a server/client model, for technology to prevent information of deposited data from being leaked from a server while a client deposits the data in the server, a method of utilizing cryptography is known. For example, in Patent Literature 1, a searching method of searching for deposited encrypted data without decrypting the encrypted data according to a request from a client is disclosed. This searching method adopts a stochastic encryption method which is securer than a decisive encryption method having one-to-one simple correspondence between plain text and a cryptograph and in which plain text and a cryptograph have one-to-m intricate correspondence. In Patent Literature 1, technique for securely searching for data deposited in a server, preventing an information leak to a server manager is disclosed.